So far the comments here only increase my confusion (which as I say,
is not bad because it means that I am learning something!).
As a Haskell newbie, the first thing I learned about monads is that
they have a type signature that creates a kind of mud you can't wash
off.
eg.
f :: String -> MyMonad String
By mentioning the monad, you get to use its special functions but as a
hard price, you must return a value with a type signature that locks
it within the monad (although you can remove the signature within
other monads using "<-").
As some people have hinted, perhaps the problem is that most Haskell
newbies are introduced to monads through the IO monad and other monads
are different.
When I plunged into Haskell earlier this year, I had no problem with
understanding static typing, higher level functions and even
separating pure functions from IO functions.
The more I learn about monads, however, the less I understand them.
I've seen plenty of comments suggesting that monads are easy to
understand, but for me they are not.
Cheers,
Kevin
On Jul 30, 12:29 pm, Tillmann Rendel
C K Kashyap wrote:
I am of the understanding that once you into a monad, you cant get out of it?
That's not correct.
There are many monads, including Maybe, [], IO, ... All of these monads provide operations (>>=), return and fail, and do notation implemented in terms of these functions, as a common interface. Using just this common interface, you cannot "get out of the monad".
But most if not all monads also provide additional operations, specific to the monad in question. Often, these operations can be used to "get out of that monad". For example, with Maybe, you can use pattern matching:
case do x <- return 5 fail "some message" return (x + 3) of Just a -> a Nothing -> 0
So we can get out of many monads, but we need to know which one it is to use the appropriate operation.
Kevin Jardine wrote:
I'm still trying to understand how monads interact with types so I am interested in this as well.
From my point of view, the most important fact about monads is:
There is nothing special about monads!
The type class Monad behaves like very other type class. A monadic type constructor behaves like every other type constructor. The type class methods (>>=), return and fail behave like every other type class method. There is nothing special about monads.
The only speciality of monads is do notation, but do notation is only a syntactic convenience, and can be translated into calls of (>>=), return and fail, which, as noted above, are not special in any way.
So, back to your question, since there is nothing special about monads, monads do not interact with types in any special way.
Tillmann _______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe